Titania-doped quartz glass and making method

ABSTRACT

Titania-doped quartz glass is manufactured by mixing a silicon-providing reactant gas and a titanium-providing reactant gas, preheating the reactant gas mixture at 200-400° C., and subjecting the mixture to oxidation or flame hydrolysis. A substrate of the glass is free of concave defects having a volume of at least 30,000 nm 3  in an effective region of the EUV light-reflecting surface and is suited for use in the EUV lithography.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2011-178758 filed in Japan on Aug. 18, 2011,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to titania-doped quartz glass suited for use inthe EUV lithography, and a method for manufacturing the glass.

BACKGROUND ART

The advanced lithography process for the fabrication of semiconductordevices favors a light source of shorter wavelength for exposure. Asubsequent transition to lithography using extreme ultraviolet (EUV) isregarded promising.

The EUV lithography uses a reflecting optical system. While EUV lighthas a short wavelength of 13.5 nm, there are available no materialshaving high transmittance at that wavelength. EUV light is reflected bya Si/Mo multilayer film sputtered on a surface of a substrate of lowcoefficient of thermal expansion (CTE) material.

Fabrication of defect-free photomasks is one of the outstanding problemsthat must be overcome before the EUV lithography can be implemented on acommercial basis. While irregularities or defects on the surface of aphotomask substrate are permissible in the KrF (wavelength 248.3 nm) andArF (wavelength 193.4 nm) lithography employing conventional dioptricsystem, defects of the same order are not negligible in the EUVlithography because of the shortness of exposure wavelength and thereflecting optical system.

The EUV lithography photomask bears defects which are generally dividedinto three types: 1) surface defects of a low CTE material substrate, 2)defects in a reflecting multilayer film, and 3) defects on a pattern. Asthe sputtering conditions are ameliorated and the cleaning technology isimproved, the number of defects on the EUV lithography photomask isreduced. However, many defects are still found on the polished surfaceof low CTE material substrate. Toward the commercial implementation ofthe EUV lithography, it is urgently required to reduce defects on thepolished surface of low CTE material substrate.

Titania-doped quartz glass is well known as the low CTE material whichis used in the reflecting optical system for the EUV lithography. Theaddition of a certain amount of titania is effective for reducing theCTE of quartz glass. Because of titania doping, however, technicaldifficulty arises in manufacturing photomask substrates which meet therequirements of defect-free and high flatness for the EUV lithographyapplication.

When titania-doped quartz glass has a non-uniform titania concentration,it is difficult to manufacture substrates having a high flatness fromsuch glass. Striae may be formed during manufacture of titania-dopedquartz glass. For example, if the reactant gas supply is inconsistent(that is, the silicon and titanium-providing reactant gases are notsupplied at constant flow rates), and/or if the temperature of thegrowth face of a titania-doped quartz glass ingot fluctuates due tovariations of the flow rates of oxygen and hydrogen gases suppliedsimultaneously, the titania concentration becomes non-uniform,generating distinct sites, known as striae. When striated substrates arepolished, irregularities are formed on the substrate surface becausestriae are different in reactivity with the polishing fluid and abrasionrate.

On the other hand, as compared with undoped quartz glass substrates,titania-doped quartz glass substrates tend to bear many defects on theirsurface after polishing and cleaning. The defects on the substratesurface may be divided into two: convex defects resulting from foreignparticles left after polishing and cleaning and high-hardness inclusionswithin quartz glass emerging at the surface; and concave defectsresulting from local high-polishing-rate inclusions within quartz glassemerging at the surface.

Titania-doped quartz glass for use as the material from which EUVlithographic members are made tends to bear more concave defects, whichinterfere with the manufacture of defect-free photomasks.

JP-A 2010-135732 and WO 2010/131662 describe silica glass substrates.Allegedly, it is preferred that concave pits of at least 60 nm be absenton the substrate surface of a surface quality area. However, the methodfor manufacturing silica glass substrates so as to eliminate concavepits of at least 60 nm is described nowhere. It is more preferred inview of mask quality that concave pits of at least 40 nm are absent onthe substrate surface of a surface quality area. However, masksubstrates in which concave pits of at least 40 nm are absent aredescribed nowhere, even in Examples. Also means for measuring defects of40 nm is described nowhere.

WO 2009/145288 describes that silica glass substrates are preferablyfree of inclusions which are believed to cause concave defects. However,it is not described even in Examples whether or not inclusions are foundin silica glass. Also means for detecting such inclusions is describednowhere.

Even in the case of titania-doped quartz glass in which no inclusionshave been found by the standard light collimating detection test,concave defects are often found on its surface after it is polished.

For the goal of defect-free polished surface, it has been a commonpractice to optimize polishing and cleaning conditions so as to reduceconvex and concave defects. Also for the purpose of reducing inclusionsin titania-doped quartz glass which cause defects on the polishedsurface, it is known in connection with the indirect manufacture oftitania-doped quartz glass to accurately control temperature conditionsduring heat treatment for homogenizing titania-doped porous silicamatrix (TiO₂—SiO₂ consolidated body), vitrification, and shaping ofTiO₂—SiO₂ glass body.

CITATION LIST

-   Patent Document 1: JP-A 2010-135732 (U.S. Pat. No. 8,012,653)-   Patent Document 2: WO 2010/131662-   Patent Document 3: WO 2009/145288

SUMMARY OF INVENTION

An object of the invention is to provide a titania-doped quartz glassused to form an EUV lithography member having a surface for reflectingEUV light, which surface is free of concave defects having a volume ofat least 30,000 nm³ and an aspect ratio of up to 10 in an effectiveregion, and a method for manufacturing the titania-doped quartz glass.

The inventors have found that a titania-doped quartz glass can bemanufactured by mixing a silicon-providing reactant gas and atitanium-providing reactant gas, preheating the reactant gas mixture at200 to 400° C., and subjecting the mixture to oxidation or flamehydrolysis with the aid of a combustible gas and a combustion-supportinggas and that the resulting titania-doped quartz glass can be used toform an EUV lithography member having a surface for reflecting EUVlight, which surface is free of concave defects having a volume of atleast 30,000 nm³ and an aspect ratio of up to 10 in an effective region.

Accordingly, the invention provides a method for manufacturing atitania-doped quartz glass, comprising the steps of mixing asilicon-providing reactant gas and a titanium-providing reactant gas,heating the reactant gas mixture at 200 to 400° C., and subjecting themixture to oxidation or flame hydrolysis with the aid of a combustiblegas and a combustion-supporting gas.

In a preferred embodiment, the method may further comprise the steps ofproviding a burner for injecting the reactant gas mixture, thecombustible gas, and the combustion-supporting gas, the burner includinga central tube for injecting the reactant gas mixture, connecting aglass or glass-lined conduit at its downstream end to the central tubefor feeding the reactant gas mixture through the glass or glass-linedconduit to the central tube, and heating and holding the reactant gasmixture at 200 to 400° C. in the glass or glass-lined conduit.

The preferred method may further comprise the steps of connecting ametal conduit to the glass or glass-lined conduit at its upstream end,and holding an upstream end portion of the glass or glass-lined conduitconnected to the metal conduit at 100 to 130° C.

In a further preferred embodiment, the burner comprises a centralmulti-fold tube section and a multi-nozzle section, the centralmulti-fold tube section including the central tube and a second tubeenclosing the central tube, the reactant gas mixture is injected throughthe central tube, and the combustion-supporting gas is injected throughthe second tube while being heated at 200 to 400° C.

In another aspect, the invention provides a titania-doped quartz glasshaving a surface where EUV light is reflected, the glass being free ofconcave defects having a volume of at least 30,000 nm³ and an aspectratio of up to 10 in an effective region of the EUV light-reflectingsurface. Preferably the titania-doped quartz glass is free ofinclusions.

In a further aspect, the invention provides an EUV lithographic membercomprising the titania-doped quartz glass defined above. The EUVlithographic member is typically an EUV lithographic photomasksubstrate.

Advantageous Effects of Invention

Since the EUV reflecting surface of a titania-doped quartz glasssubstrate is free of concave defects having a volume of at least 30,000nm³, the titania-doped quartz glass is suited for use in the EUVlithography, especially as EUV lithographic photomask substrates.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates an exemplary reactant gas mixtureheating mechanism.

FIG. 2 is a transverse section of a gas injection outlet of a burnerused for the manufacture of titania-doped quartz glass in Examples.

DESCRIPTION OF EMBODIMENTS

One embodiment of the invention is a titania-doped quartz glass having asurface where EUV light is reflected, the glass being free of concavedefects having a volume of at least 30,000 nm³ in an effective region ofthe EUV light-reflecting surface. Preferably, the titania-doped quartzglass is free of concave defects having a volume of at least 20,000 nm³,more preferably at least 15,000 nm³.

Using the titania-doped quartz glass free of concave defects in aneffective region of the EUV light-reflecting surface as a photomasksubstrate, a photomask free of line cuts or short-circuits, havingimproved CD uniformity, and suited for EUV lithography is obtainable.

The term “effective region” of the EUV light-reflecting surface refersto a surface region of an EUV lithographic member where EUV light isreflected. Specifically in the case of a photomask, it refers to aregion where a circuit to be transferred to a silicon wafer is written.In the case of a photomask substrate of 152.4 mm×152.4 mm squares, itgenerally refers to a central region of 142 mm×142 mm squares within thephotomask substrate.

In the one embodiment, the titania-doped quartz glass is also free ofconcave defects having an aspect ratio of up to 10. Those concavedefects having an aspect ratio in excess of 10 are introduced by thepolishing and cleaning steps of titania-doped quartz glass, rather thanoriginating from the bulk of titania-doped quartz glass. It is notedthat the aspect ratio is a ratio of major side to minor side of aconcave defect in the member surface.

In a preferred embodiment, the titania-doped quartz glass is free ofinclusions because the presence of inclusions can be a cause of concavedefects in the polished surface.

Another embodiment is a method for manufacturing titania-doped quartzglass free of concave defects having a volume of at least 30,000 nm³ inan effective region of the EUV light-reflecting surface, ortitania-doped quartz glass free of inclusions. The desired glass ismanufactured by mixing a silicon-providing reactant gas and atitanium-providing reactant gas, heating the reactant gas mixture at 200to 400° C., and feeding the mixture to a synthetic quartz burner wherethe mixture is subjected to oxidation or flame hydrolysis with the aidof a combustible gas and a combustion-supporting gas. Although thereason why the step of preheating the reactant gas mixture at a hightemperature results in minimization of concave defects in the surface isnot well understood, it is believed that the previous exposure of thereactant gas mixture to high temperature improves the mixing evennessand reactivity of reactant gases, allows the mixed reactant gases toundergo reaction immediately after injection from the burner, and thuseffectively contribute to formation of silica-titania fine particleswhich are uniform in both composition and particle size.

In order that the reactant gas mixture be heated and held at a hightemperature of 200 to 400° C., the reactivity of a material of a gasconduit with the reactant gases must be taken into account. A glass orglass-lined conduit is preferably used as the conduit where the reactantgas mixture is held at a high temperature. Specifically, quartz glass isused as the glass or glass-lined conduit to avoid contamination withincidental impurities. While the glass or glass-lined conduit serves toheat and hold the reactant gas mixture at a high temperature of 200 to400° C., the temperature of a portion (or upstream end side) of theglass or glass-lined conduit which is disposed near the connection to ametal conduit, typically stainless steel conduit, is preferably loweredto the normal conduit heating temperature of 100 to 130° C. If thetemperature of the metal conduit connected to the glass or glass-linedconduit is elevated by heat transfer from the glass or glass-linedconduit, then the metal conduit can be corroded with the reactant gasmixture, which can cause introduction of impurities, eventual generationof inclusions within the titania-doped quartz glass, and formation ofconcave defects.

When the reactant gas mixture is held at a high temperature of 200 to400° C., preferably the mixture is heated immediately before entry intothe synthetic quartz burner. A shortened distance from this heating zoneto the injection outlet of the synthetic quartz burner for creating thereaction site of the reactant gas mixture makes it easy to keep thetemperature of the reactant gas mixture until the reaction site isreached. In this sense, the glass or glass-lined conduit for feeding,heating and holding the reactant gas mixture at a high temperature of200 to 400° C. is preferably connected directly to the central tube ofthe burner for injecting the reactant gas mixture.

Where a filter is interposed in a reactant gas mixture feed line,preferably the glass or glass-lined conduit for feeding, heating andholding the reactant gas mixture at a high temperature of 200 to 400° C.is positioned immediately downstream of the filter.

FIG. 1 schematically illustrates one exemplary reactant gas mixtureheating mechanism 1. The heating mechanism 1 includes one end portion orupstream end portion 2 b which is coupled to a metal conduit section 3.The metal conduit section 3 has a filter 4 interposed near itsdownstream end. The heating mechanism 1 includes another end portion ordownstream end portion which is coupled to a central tube 11 of a burnerfor injecting a reactant gas mixture. The heating mechanism 1 isconstructed by a glass conduit or glass-lined conduit. The glass orglass-lined conduit includes a hot heating portion 2 a (delineated bythe two-dots-and-dash line) and an upstream end portion 2 b (delineatedby the dot line) coupled to the metal conduit 3. Preferably the hotheating portion 2 a (also referred to as heating zone I) is heated at ahigh temperature of 200 to 400° C., whereas the upstream end portion 2 b(also referred to as heating zone II) is heated at an ordinary conduitheating temperature of 100 to 130° C. It is acceptable that the upstreamend portion 2 b (or heating zone II) be heated at a temperature of 200to 400° C. as well.

The reactant gas mixture used herein is a mixture of a silicon-providingreactant gas and a titanium-providing reactant gas. If desired, acombustion-supporting gas such as oxygen gas may be admixed in thismixture.

The silicon-providing reactant gas used herein may be selected fromwell-known organosilicon compounds, for example, silicon tetrachloride,chlorosilanes such as dimethyldichlorosilane and methyltrichlorosilane,and alkoxysilanes such as tetramethoxysilane, tetraethoxysilane, andmethyltrimethoxysilane.

The titanium-providing reactant gas used herein may also be selectedfrom well-known compounds, for example, titanium halides such astitanium tetrachloride and titanium tetrabromide, and titanium alkoxidessuch as tetraethoxytitanium, tetraisopropoxytitanium,tetra-n-propoxytitanium, tetra-n-butoxytitanium,tetra-sec-butoxytitanium, and tetra-t-butoxytitanium.

A mixing proportion of silicon-providing reactant gas andtitanium-providing reactant gas may be determined in accordance with thedesired titania content of titania-doped quartz glass. It is noted thatthe titania content of titania-doped quartz glass is preferably 4 to 12%by weight, more preferably 5 to 10% by weight. Where acombustion-supporting gas is admixed in the reactant gas mixture, amolar amount of the combustion-supporting gas is preferably at least 8times, more preferably at least 10 times the total moles of silicon andtitanium-providing reactant gases.

According to the invention, titania-doped quartz glass may be preparedby feeding the reactant gas mixture, a combustible gas and acombustion-supporting gas to a burner. The burner typically of syntheticquartz preferably comprises a central multi-fold tube section and amulti-nozzle section. The central multi-fold tube section includes areactant gas injecting nozzle at the center and a plurality ofconcentrically arranged nozzles. The plurality of nozzles receivecombustion-supporting gas or combustible gas. On the other hand, themulti-nozzle section includes small-diameter nozzles arranged in rowsconcentric with respect to the central reactant gas injecting nozzle forinjecting combustion-supporting gas and a space outside thesmall-diameter nozzles for injecting combustible gas. In a preferredembodiment of the invention, the gas flowing through the nozzle (secondtube) arranged adjacent to the central reactant gas injecting nozzle isheated to an equivalent temperature to the reactant gas mixture. Sinceit is difficult from the structural aspect to heat the nozzles of thesynthetic quartz burner independently, the temperature of the gasflowing through the second tube which is lower than the reactant gasmixture can lower the temperature of the heated reactant gas mixture.

Specifically, the preferred burner is of the structure shown in FIG. 2.The burner 1 is illustrated in FIG. 2 as comprising a central multi-foldtube section A at the center and a multi-nozzle section B enclosingsection A. The central multi-fold tube section A includes a central tube(or nozzle) 11 for injecting the reactant gas mixture, a firstcombustion-supporting gas feed tube 12 enclosing the central tube 11, afirst combustible gas feed tube 13 enclosing the tube 12, a secondcombustion-supporting gas feed tube 14 enclosing the tube 13, and asecond combustible gas feed tube 15 enclosing the tube 14, in aconcentric telescopic arrangement. The multi-nozzle section B includes afirst shell 16 disposed outside and enclosing the second combustible gasfeed tube 15, and a second shell 17 disposed outside and enclosing thefirst shell 16. A multiplicity of third combustion-supporting gas feedtubes 18 are disposed between the second combustible gas feed tube 15and the first shell 16, in five rows concentric with the central tube 11while combustible gas is fed through the remaining space within thefirst shell 16 (outside third combustion-supporting gas feed tubes 18).A multiplicity of fourth combustion-supporting gas feed tubes 19 aredisposed between the first and second shells 16 and 17 in a concentricrow while combustible gas is fed through the remaining space within thesecond shell 17 (outside fourth combustion-supporting gas feed tubes19).

In the preparation of titania-doped quartz glass according to theinvention, the central multi-fold tube section A of the burnerpreferably includes at least three tubes, and more preferably at leastfive tubes. A multiplicity of third combustion-supporting gas feed tubes18 in the multi-nozzle section B are disposed preferably in five rows,more preferably in six rows concentric with the central multi-fold tube.

The combustible gas used herein may be hydrogen or hydrogen-containinggas, optionally in combination with another gas such as carbon monoxide,methane or propane. The combustion-supporting gas used herein may beoxygen or oxygen-containing gas.

In the method for the preparation of titania-doped quartz glass, thereactant gas mixture is preferably fed at a linear velocity of at least55 m/sec, more preferably 60 to 100 m/sec. This is because the reactantgas mixture is kept at the high temperature and thus highly reactive. Ifthe linear velocity of the reactant gas mixture is slow, silica-titaniafine particles created may deposit on the burner tip and scattertherefrom, becoming inclusions in titania-doped quartz glass and causingconcave defects.

In the method for the preparation of titania-doped quartz glass, oxygengas as the combustion-supporting gas and hydrogen gas as the combustiblegas are fed through the multi-nozzle section and the central multi-foldtube section of the burner. Preferably, oxygen in excess of thestoichiometry, specifically in the range: 1.7≦H₂/O₂ ratio<2, isavailable in at least one, more preferably both of the multi-nozzlesection and the central multi-fold tube section. If hydrogen in excessof the stoichiometry (specifically H₂/O₂ ratio≧2) is available in boththe multi-nozzle section and the central multi-fold tube section, thentitania-doped quartz glass can be colored and at the same time,microcrystalline titanium oxide is likely to form.

In another preferred embodiment, the combustible gas, typically hydrogengas is injected through the burner at a linear velocity of less than orequal to 100 m/sec, more preferably less than or equal to 90 m/sec. Ifthe linear velocity of hydrogen gas injected as the combustible gasthrough the burner is higher than 100 m/sec, the titania-doped quartzglass prepared under such conditions may have the risk of thermalhysteresis on use as the EUV lithography member. The linear velocity ofthe combustible gas, typically hydrogen gas is usually at least 0.5m/sec, and preferably at least 1 m/sec though the lower limit is notcritical.

According to the invention, titania-doped quartz glass may be preparedby feeding a combustible gas containing hydrogen and acombustion-supporting gas containing oxygen to a burner built in aquartz glass-manufacturing furnace, burning the gases to form anoxyhydrogen flame at the burner tip, feeding a silicon-providingreactant gas and a titanium-providing reactant gas through the burnerinto the flame for subjecting the reactant gases to oxidation or flamehydrolysis to thereby form silica, titania and composite fine particles,depositing the fine particles on a target horizontally disposed forwardof the burner, and concurrently melting and vitrifying the depositedparticles to grow titania-doped quartz glass to form an ingot, hotshaping the ingot into a predetermined shape, and annealing and slowlycooling the shaped ingot. This is known as the horizontal directprocess.

If the vertical direct process is used, it is difficult due to thefurnace structure to avoid fragments of insulator and other membersdisposed above the growth face of a titania-doped quartz glass ingotfrom spallling off and depositing on the ingot growth face. Suchfragments can become inclusions in titania-doped quartz glass and causeconcave defects on the surface.

Also, when the indirect process is used for the preparation oftitania-doped porous silica matrix, it is relatively easy to avoidcontamination of the matrix with foreign particles. However, bubbles areoften left after vitrification of titania-doped porous silica matrix.The bubbles can be apparently extinguished during subsequent heattreatment, typically hot shaping of titania-doped quartz glass. However,the extinction occurs as a result of the gas in bubbles being dissolvedin titania-doped quartz glass, indicating that structurally sparseregions are formed within the titania-doped quartz glass. Such sparseregions in titania-doped quartz glass allow for a higher machining rateduring polishing and are thus prone to cause concave defects.

During preparation of titania-doped quartz glass, that is, titania-dopedporous silica matrix, it is desired to avoid contamination of the silicamatrix with foreign particles. To this end, air to be fed to the furnacemust be passed through a filter beforehand. Additionally, a vent ispreferably provided on an extension in the direction of injection ofsilicon and titanium-providing reactant gases through the syntheticquartz burner, so that titania-doped silica fine particles may notdeposit on the furnace inner wall.

During preparation of titania-doped quartz glass, the target istypically rotated at a rotational speed of at least 5 rpm, preferably atleast 15 rpm, and more preferably at least 30 rpm. This is becausestructurally or compositionally non-uniform zones (like striae andstrains) which are undesired on use of titania-doped quartz glass as EUVlithographic members generate, depending largely on the unevenness oftemperature in a portion where titania-doped quartz glass grows on therotating target. Then the generation of structurally or compositionallynon-uniform zones in titania-doped quartz glass can be inhibited byincreasing the rotational speed of the target so that an eventemperature may be available in a portion where titania-doped quartzglass grows.

Preferably the titania-doped quartz glass ingot thus prepared is thenheat treated at a temperature of 700 to 1,150° C. for at least 50 hours,thereby removing hydrogen molecules from within the glass. Specificallytitania-doped quartz glass has such a hydrogen molecule concentrationthat the peak near 4,135 cm⁻¹ attributable to hydrogen molecule is belowthe detection limit on measurement by Raman spectroscopy using aspectrometer NRS-2100 (JASCO Corp.) and a 4-W argon ion laser as anexcitation light source. This is because titania-doped quartz glasscontaining more hydrogen molecules tends to generate bubbles thereinwhen it is hot shaped into the desired shape.

In order that the titania-doped quartz glass ingot be shaped into adesired shape suited for a particular EUV lithography member such as amirror, stage or photomask substrate, it is hot shaped at a temperatureof 1,500 to 1,800° C. for 1 to 10 hours. After the hot shaping, thetitania-doped quartz glass is annealed. Annealing may be conducted underwell-known conditions, for example, by holding at a temperature of 700to 1,300° C. in air for 1 to 200 hours. This may be followed by slowcooling. Although slow cooling for titania-doped quartz glass isgenerally down to about 500° C., the invention prefers slow cooling downto 300° C., more preferably down to 200° C. The slow cooling rate ispreferably 1 to 20° C./hr, more preferably 1 to 10° C./hr.

After the annealing/slow cooling treatment, the titania-doped quartzglass is processed into a predetermined size by machining or slicing andthen polished by a double-side lapping machine with an abrasive such assilicon oxide, aluminum oxide, molybdenum oxide, silicon carbide,diamond, cerium oxide or colloidal silica, thereby forming an EUVlithography member. The polishing method of WO 2009/150938 is preferablyemployed herein in order to inhibit concave defects from generatingduring double-side lapping.

For observation of concave defects on the polished surface, a lightsource having the exposure wavelength (λ=13.5 nm) of the EUV lithographyis used because fine size defects must be measured. Conventional flawdetectors using visible light and UV light are difficult to detectdefects of the size contemplated herein. Since titania-doped quartzglass has a low reflectance at the exposure wavelength of the EUVlithography, a reflective multilayer film is previously deposited on thesurface by sputtering. The reflective multilayer film is typically astack of Si layers of 4.5 nm thick and Mo layers of 2.3 nm alternatelydisposed in 5 periodicities. Concave defects are observed by monitoringreflecting light from the reflective multilayer film on the surface. Ifa concave defect is present on the surface, then the reflectivemultilayer film is deformed conformal to the defect configuration, andthus the reflecting light from the reflective multilayer film providessubstantially the same signal as the signal directly from the defect.

Concave defects are measured by irradiating EUV light from an EUV lightsource (Energetiq Technology, Inc.) to the reflective multilayer film ona titania-doped quartz glass member, collecting reflected light througha Schwarzschild optical system having a magnification of 20×, andsensing it by a CCD camera. It is noted that concave defect measurementis conducted as dark field observation, and an effective region in theEUV light reflecting surface of the EUV lithography member is scan inentirety. The reflective multilayer film at the position of the memberwhere a signal assigned to a defect is obtained is observed under AFM todetermine its geometry and topography, from which the volume of thedefect is computed and reported as the volume of a concave defect on thesurface. Also an aspect ratio is determined from the defect geometryobtained by AFM observation.

Inclusions are measured by coupling a visible light source with a spotlight source (Hamamatsu Photonics Co., Ltd.) and scanning the effectiveregion of the EUV light reflecting surface in entirety. The visiblelight irradiated site is magnified and observed under an opticalmicroscope to see whether or not inclusions including bubbles,crystallized sites and local refractive index variations (correspondingto variations of composition, e.g., TiO₂ concentration or OH groupconcentration, local variations of glass structure, and the like) arepresent. Also, by similar scanning of the effective region in entiretyaside from using a UV light source (250 nm enhancement mode), andsimultaneous optical microscope observation, any fluorescent colorchange of titania-doped quartz glass due to contamination withimpurities is observed.

Example

Examples and Comparative Examples are given below for furtherillustrating the invention although the invention is not limitedthereto.

Example 1

A titania-doped quartz glass ingot was prepared by placing a quartzglass burner as shown in FIG. 2 relative to a target, feeding gases(SiCl₄, TiCl₄, O₂, H₂) to respective nozzles of the burner as shown inTable 1, forming an oxyhydrogen flame, effecting oxidation or flamehydrolysis of silicon tetrachloride and titanium tetrachloride in theoxyhydrogen flame to produce SiO₂ and TiO₂, depositing silica andtitania fine particles on the target, and concurrently melting andvitrifying the particles. The target was disposed forward of the burnerat a distance 280 mm and an angle of 128° relative to the burner. Duringthe process, the target was rotated at 50 rpm and retracted at 10 mm/hr.A reactant gas mixture heating mechanism 1 including heating zones I andII as shown in FIG. 1 was installed immediately upstream of the burnerand immediately downstream of a reactant gas mixture filter 4. Theheating zones I and II were held at 375° C. and 125° C., respectively.Also the oxygen gas flow through the second tube of the centralmultifold tube section of the burner was held at 375° C. A stainlesssteel conduit was used as the metal conduit 3 and held at 125° C.

The resulting titania-doped quartz glass ingot was heat treated at1,100° C. for 100 hours to remove hydrogen from within the glass.Samples were taken from opposite ends of the ingot and analyzed by Ramanspectroscopy, but peaks assigned to hydrogen molecule were not observed.The titania-doped quartz glass ingot was hot shaped into a square columnof 160 mm×160 mm by heating at 1700° C. for 6 hours. The column wassliced into substrates of 7 mm thick. The substrates were annealed in afurnace lined with high-purity porous silicon carbide insulator byholding in air at 850° C. for 150 hours and then slowly cooled at a rateof 2° C./hr to 200° C. The substrates were ground on edge surfaces to asquare shape of 152.4 mm×152.4 mm whereupon they were further polished,cleaned and dried in accordance with Example 1 of WO 2009/150938. Aninclusion inspection test was carried out on these substrates usingvisible and UV light sources.

After the titania-doped quartz glass substrates were cleaned and driedagain, a reflective multilayer film was deposited thereon. Concavedefects on the surface covered with the reflective multilayer film weremeasured.

Example 2

Titania-doped quartz glass was prepared under the same conditions as inExample 1 except that the temperature of heating zone I of the reactantgas mixture heating mechanism 1 was 300° C.

Example 3

Titania-doped quartz glass was prepared under the same conditions as inExample 1 except that the temperature of heating zone I of the reactantgas mixture heating mechanism 1 was 220° C. and the oxygen gas flowthrough the second tube of the burner central multifold tube section washeld at room temperature (20° C.).

Example 4

Titania-doped quartz glass was prepared under the same conditions as inExample 1 except that the temperature of heating zone II of the reactantgas mixture heating mechanism 1 was 375° C.

Comparative Example 1

The reactant gas mixture heating mechanism was removed from the systemof Example 1. That is, the stainless steel conduit was directly coupledto the burner. The stainless steel conduit was held at a temperature of125° C. The oxygen gas flow through the second tube of the burnercentral multifold tube section was held at room temperature (20° C.) Theremaining conditions were the same as in Example 1.

Comparative Example 2

A titania-doped quartz glass ingot was prepared by using a quartz glassburner as shown in FIG. 2 and feeding gases (SiCl₄, TiCl₄, O₂, H₂) torespective nozzles of the burner as shown in Table 1. The remainingconditions were the same as in Example 1.

TABLE 1 Cross- Gas flow rate, Nm³/hr (reactant sec- gas linear velocity,m/sec) tional Example 1 to 4, Compar- area, Comparative ative Gas mm²Example 1 Example 2 Central 1st tube SiCl₄ 10.18 1,420 g/hr 1,150 g/hrmulti- TiCl₄   190 g/hr   150 g/hr fold O₂ 2.25 1.65 tube (67.1) (49.6)section 2nd tube O₂ 19.47 0.60 0.55 3rd tube H₂ 49.46 15.00 14.50 4thtube O₂ 37.18 9.00 8.00 5th tube H₂ 37.04 5.20 4.50 Multi- Inside nozzleO₂ 241.15 12.50 11.00 nozzle Inside shell H₂ 6161 25.00 21.00 sectionOutside O₂ 88.42 5.00 4.00 nozzle Outside shell H₂ 2286 6.00 5.00

TABLE 2 Comparative Example Example 1 2 3 4 1 2 Concave 15,000 ≦ V <20,000 0 0 16 94 225 428 defects 20,000 ≦ V < 25,000 0 0 5 22 102 228(volume V, 25,000 ≦ V < 30,000 0 0 0 4 38 152 nm³) 30,000 ≦ V 0 0 0 0 2588 Inclusions Visible light nil nil nil nil nil found observation UVobservation nil nil nil found nil nil

Japanese Patent Application No. 2011-178758 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A method for manufacturing a titania-doped quartz glass, comprisingthe steps of: mixing a silicon-providing reactant gas and atitanium-providing reactant gas, heating the reactant gas mixture at 200to 400° C., and subjecting the mixture to oxidation or flame hydrolysiswith the aid of a combustible gas and a combustion-supporting gas. 2.The method of claim 1, further comprising the steps of: providing aburner for injecting the reactant gas mixture, the combustible gas, andthe combustion-supporting gas, the burner including a central tube forinjecting the reactant gas mixture, connecting a glass or glass-linedconduit at its downstream end to the central tube for feeding thereactant gas mixture through the glass or glass-lined conduit to thecentral tube, and heating and holding the reactant gas mixture at 200 to400° C. in the glass or glass-lined conduit.
 3. The method of claim 2,further comprising the steps of: connecting a metal conduit to the glassor glass-lined conduit at its upstream end, and holding an upstream endportion of the glass or glass-lined conduit connected to the metalconduit at 100 to 130° C.
 4. The method of claim 1, wherein the burnercomprises a central multi-fold tube section and a multi-nozzle section,the central multi-fold tube section including the central tube and asecond tube enclosing the central tube, the reactant gas mixture isinjected through the central tube, and the combustion-supporting gas isinjected through the second tube while being heated at 200 to 400° C. 5.A titania-doped quartz glass having a surface where EUV light isreflected, the glass being free of concave defects having a volume of atleast 30,000 nm³ and an aspect ratio of up to 10 in an effective regionof the EUV light-reflecting surface.
 6. The titania-doped quartz glassof claim 5 which is free of inclusions.
 7. An EUV lithographic membercomprising the titania-doped quartz glass of claim
 5. 8. The EUVlithographic member of claim 7 which is an EUV lithographic photomasksubstrate.